The present invention relates generally to semiconductor device manufacturing, and, more particularly, to a method for transmission electron microscopy (TEM) sample preparation for electron holography.
Advancements in Transmission Electron Microscopy (TEM) technology enable materials to be analyzed at near atomic resolution by providing high-magnification, high-resolution imaging and analysis capabilities. TEM enables scientists to gather information relating to a material's physical properties, such as its microstructure, crystalline orientation and elemental composition. This information has become increasingly important as the need for advanced materials for use in areas such as microelectronics and optoelectronics, biomedical technology, aerospace, transportation systems and alternative energy sources, among others, increases.
TEM is accomplished by examining material specimens under a transmission electron microscope. In a transmission electron microscope, a series of electromagnetic lenses direct and focus an accelerated beam of electrons, emitted from an electron gun contained within the microscope, at the surface of a specimen. Electrons transmitted through the specimen yield an image of the specimen's structure, which provides information regarding its properties. In addition, elemental and chemical information is provided by both the transmitted electrons and the x-rays that are emitted from the specimen's surface as a result of electron interaction with the specimen.
In 1947, a Hungarian-British physicist named Dennis Gabor sought to find a way to sharpen the resolution of the images initially produced in transmission electron microscopes, which were in their infancy at the time. He proposed electron holography, a method of interference imaging in which the phase and amplitude components of the electron beam are separated to correct the spherical aberration of the microscope. In this regard, the electron beam source is split into the incident, undeviated electron wave (i.e., the reference wave) and the image wave (or object wave) diffracted by the specimen and exiting the bottom surface thereof. Assuming the electron optical geometry is correctly set up, these two waves can be made to interfere. The resulting interference pattern is then processed using optical techniques to form the holograms (images).
Unfortunately, the electron microscopes of Gabor's era did not produce an electron wave with sufficient coherence to permit the proper degree of interference required to make a useful hologram. More recently however, the development of TEMs using highly coherent field-emission electron sources has made electron holography a more effective undertaking. This technique has been shown to be particularly valuable for two-dimensional, p-n junction potential mapping of semiconductor devices with high spatial resolution. Such information is valuable for semiconductor device development and yield improvement.
Before a specimen can be analyzed using TEM (including electron holography), it must be prepared using various techniques to achieve the necessary electron transparency as it is necessary for the electron beam to transmit through the specimen. This electron transparency is accomplished by thinning a defined area of the specimen. For equal resolution, the required thickness of the specimen is dependent on the accelerating voltage of the transmission electron microscope. For example, using a 120 kV microscope, the specimen thickness should be on the order of about 100 to about 2000 angstroms (Å). In contrast, A 1,000 kV microscope can tolerate a specimen thickness of up to about 5,000 Å.
Specimens are prepared through several well-known methods, including, but not limited to, electrolytic thinning, mechanical polishing, ultramicrotomy, crushing, and ion milling. Often times, multiple methods are utilized to prepare a single specimen. Normal TEM sample preparation utilizes a deposited material such as tetraethyl orthosilicate (TEOS) on top of the sample in order to protect it from cracking and rounding during a subsequent polishing operation. If the sample becomes rounded, then subsequent ion milling may cause re-deposition of material on the shadow region near the top surface. This in turn results in a rough surface formed on the sample and leads to a noisy phase map for electron holography.
However, for electron holography, any protective material formed over the sample would need to be removed prior to the imaging, due to the requirement of having a vacuum region near the area of interest for a reference electron wave to pass. A conventionally deposited material such as TEOS cannot be removed (i.e., etched away) without attacking the sample itself. Accordingly, it would be desirable to be able to provide a protective layer during electron holography sample preparation in a manner that also allows for the removal of the protective layer prior to imaging.